BACKGROUND:Little is known about how to facilitate participation in physical activity among children receiving acute cancer treatment. OBJECTIVE:To understand the parental perspectives on physical activity for children during acute cancer treatment and explore strategies to overcome physical inactivity. METHODS:A qualitative study was completed. Data were collected via semistructured interviews with parents of children (aged 4-18 years) who were in their first nine months of cancer treatment. Data were analyzed thematically. RESULTS:Twenty parents were interviewed. A childhood cancer diagnosis and subsequent treatment were described as setting in motion a spiral of physical inactivity. Parents identified movement restrictions as a result of commencing treatment and the hospital environment as factors initiating this decline. Parents described the subsequent impact of movement restrictions on their child over time including loss of independence, isolation, and low motivation. These three consequences further contributed to an inability and unwillingness to be physically active. Parents responded in a variety of ways to their child's inactivity, and many were motivated to overcome the barriers to physical activity yet exhibited a reduced capacity to do so. Suggested intervention strategies highlighted the need for comprehensive support from the organization providing treatment. CONCLUSIONS:Reasons for reduced physical activity in children receiving acute treatment for cancer are complex and multifactorial. Inactivity cannot be addressed by children and parents alone but requires support from the oncology team through changes to the environment, services, and policies to promote physical activity. These findings may be used to inform targeted, effective, and feasible physical activity interventions.